To improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing the ratio of specific heat. Generally speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is a limit, though, at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean burn limit include improving ignitability of the mixture by enhancing the fuel preparation, for example, using atomised or vaporized fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture.
Another method for operating an engine with a very lean or diluted air/fuel mixture is combustion by auto-ignition, or homogenous charge compression ignition (HCCI). HCCI mode is an engine operation state in which a substantially homogenous charge of fuel and air is compressed by a piston and ignites automatically.
In HCCI mode, when certain conditions are met within a homogenous charge of lean air/fuel mixture during low load, homogenous charge compression ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge. The temperature is increased by the compression until the entire charge reacts simultaneously. Although not necessary for ideal HCCI combustion, a spark can be used to extend the operational window and stabilize the combustion.
Moreover, in HCCI mode there is no moving flame front like in the spark ignition (SI) mode. Since the ignition occurs at several places at the same time and the charge burns simultaneously, the heat release rate is increased.
The lack of a single flame front reduces temperature and increases the heat release rate, thus increasing the thermal efficiency of the combustion. Moreover, the fact that the mixture can be extremely lean and diluted due to the lack of a flame front increases the pumping and thermal efficiency.
Since a diesel engine in a stratified compression ignition (CI) mode also ignites by auto-ignition, HCCI mode is known as an attempt to make a SI engine work like a more efficient diesel engine. Compared with HCCI mode, CI mode has a substantially stratified charge where ignition occurs at the boundary of unmixed fuel which is injected to initiate combustion. Because the engine injects the fuel at least during the intake phase in HCCI mode, which is earlier than in a CI-mode, the fuel and air will have more time to mix and the charge will consequently be (more) homogenous when ignited.
Furthermore, HCCI mode is an attempt to achieve SI mode like emissions along with the efficiency of CI mode. For example, in the HCCI mode the engine can work in homogenous lean and/or diluted operation. The homogenous lean charge of fuel and air leads to a cleaner combustion and lower emissions. The propagating flame in SI mode and the stratified charge in CI mode result in a highly heterogeneous burnt gas temperature within the charge, thus resulting in very high local temperature values. This, for instance, creates high NOx emissions. The homogenous combustion uniformly distributed throughout the charge in the HCCI from many ignition sites results in lower temperature values and extremely low NOx emission.
In some parts of the operation window, the HCCI combustion mode has the disadvantage of higher carbon monoxide (CO) and hydrocarbon (HC) emissions compared to SI and CI mode. Other disadvantages include high peak pressure and limited power range.
A further disadvantage with the HCCI mode is the difficulty to control the timing of the auto-ignition. The location (e.g., timing) of the peak pressure, which is related to auto-ignition timing, needs to be controlled to take place between approximately 4 and 8 degrees after piston top dead center (TDC) of the compression stroke. This is to optimize fuel consumption and emissions, and to avoid knocking combustion or partial burn/misfire. Also, NOx emissions will be significantly lower if the timing control of the auto-ignition is working.
In SI mode, the spark controls the ignition timing while, in CI mode, the fuel injected into compressed air initiates the ignition. In HCCI mode, the mixture of fuel and air will auto-ignite whenever certain conditions are met, such as when the temperature is sufficiently high; thus, there is no initiator such as a spark or fuel injection to start the combustion.
One main control parameter used is control of the timing of the valves, which is known as variable valve timing (VVT). In VVT, the timing of the intake and/or exhaust valves can be changed during operation. The valves may be controlled in different ways, for example, by using independently controlled valves or by controlling the camshaft angle, which is known as variable camshaft timing (VCT).
Variable valve timing can, for instance, control the amount of residuals captured in the chamber by changing the valve overlap. It is believed that the high proportion of burnt gases remaining in the chamber from the previous combustion is responsible for providing the hot charge temperature and active fuel radicals necessary to promote HCCI in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, HCCI is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, for example, by said controlling of the timing of the intake and exhaust valves or by re-circulating the exhaust gases.
One example of a VVT system for increasing the proportion of burnt gases in the chamber is disclosed in US 2002/0134333. The document discloses an exhaust gas recirculation (EGR) passage that re-circulates exhaust gases in an exhaust pipe of an engine to an intake system by negative intake-gas pressure. An intake cam that opens and closes an intake valve is provided so that a phase angle of the intake cam may be adjusted. The phase angle of the intake cam is adjusted in accordance with an increase or decrease of a flow rate of recirculation of exhaust gases. The valves of all cylinders are controlled at the same time instead of individually.
Another example of a VVT system for increasing the proportion of burnt gases in the chamber is described in document EP 1435 442. During the compression ignition mode, the exhaust valve (EV) is adapted to be closed before TDC during the exhaust stroke of the piston while the intake valve (IV) is opened after TDC during the same stroke. This is called a negative valve overlap (NVO) and occurs during the gas exchange phase. Thereby, residual exhaust gases are kept in the chamber which increases the heat in the chamber and consequently initiates the auto-ignition in the compression stroke.
EP 1435 442 also describes the use of at least one pilot injection which is injected before TDC during the NVO and, further, at least one main injection injected after TDC during the NVO but before TDC of the compression stroke. The fuel from the pilot injection will react in the residual exhaust gas, forming radicals, intermediates or combustion products. This reaction can be exothermic, hence, heating the residuals, resulting in earlier timing of the auto-ignition temperature.
In order to control these injections, EP 1435 442 describes the use of at least one sensor to measure engine operation parameters including a combustion chamber pressure sensor, an intake manifold pressure sensor and a λ-probe in the exhaust conduit, as well as temperature sensors for intake air, engine coolant and engine oil. Knocking is detected by measuring the peak pressure and/or pressure variations in the chamber. A controller evaluates the signals from sensors that indicate knock and combustion stability. A knock signal is deemed to be high if the filtered peak pressure during combustion exceeds an expected pressure level. When a coefficient of variation (COV) signal is high, this is indicated by variation of location peak pressure.
The ability to control the combustion phasing of an HCCI internal combustion engine is vital. The location (i.e., timing) of the peak pressure (approximately 50% burnt angle) needs to be controlled, for example, between 4 and 8 degrees after TDC. This is to avoid knocking sound which damages the engine in the long run. This is also to avoid combustion instability or high indicated mean effective pressure coefficient of variation (IMEP COV). Also, NOx emissions will be significantly lower if the control is working.